1. Field of the Invention
The invention relates to gas separation. More specifically, the invention relates to a dust removal apparatus for combustion products and other gases. An upflow or downflow, up-wash graded sand filter is disclosed.
2. Description of the Prior Art
Sand or gravel bed filters are well known as filtering devices for gases, usually hot gases. Several configurations of sand-bed and gravel-bed filters are well known, including down-flow filters and up-flow filters. In the industry a prominent design of down-flow filters is referred to as a "gravel-bed filter."
Down-flow filters represent the simplist form, consisting of a tank containing a filtering medium such as sand or gravel and having a top inlet for dust-laden gases and a bottom outlet for filtered gas. Where the filtering medium is sand and gravel, the sand and gravel will be of various sizes. To remove the accumulated dirt from the sand and gravel, a washing or wash stage is conducted wherein gas is blown up through the bed and at the same time the upper portion of the filter media is raked by rabble arms to lift the sand and gravel and roll it around. At the conclusion of washing, the sand settles in its mixed state and, accordingly, is not graded in the filter with the fastest settling particles at the bottom and the slowest settling particles at the top. When the filtering operation is taking place, the finest particles in the gas which can be filtered out by the size of media present are filtered out by immediate contact with the finest particles of sand mixed with the coarser gravel at the top of the filter, and the remaining solids merely build up on the top of the filter. Very little material is caught in the rabbled portion of the bed of sand under these conditions.
Coarse sand or small gravel does not remove fine dust particles; in commercial gravel bed filters no fine media is used, the fine particles of dust are not removed by the filter, only coarser dust particles are removed by the filter, and because the filter media is uniformly coarse, the coarser dust particles are removed inside the gravel bed and less so at the surface, although most of the dust removed is trapped in the upper few inches of the bed.
An operating design of graded coarse to fine up-flow sand-bed filter is used to remove radioactive particles from exit gases from certain nuclear operations. Normally the bed in this filter is not washed, and is abandoned after it has become loaded with fine particles. (G. H. Sykes and J. A. Harper, "Design and Operation of a Large Sand Bed for Air Filtration," In Treatment of Air-Borne Radioactive Wastes, Intl. Atomic Energy Agency, Vienna, 1968, pp 215,220). The top filter media is fine sand, the dust particles uniformly are fine so that no bridging occurs at the inlet surface of the filter media, and the finest of dust particles are trapped and are removed in the bed of sand. Superficial air velocities in this dust filter are 4.7 feet per minute. To prevent the 30 to 50 mesh sand filter bed from being blown out of the collector, the media is covered with and weighted down by 6" of 4 to 8 mesh gravel and 6" of 1/4" to 1/8" gravel. Tests have been conducted in which uniformly fine dust particles have been filtered out in sand-bed filters. No coarse dust particles were present, and because of the absence of coarser dust particles to bridge the interstices of the filter media, the initial surface of the filter media did not become plugged, and the fine dust particles were trapped throughout the bed of filter media. These fine-sand filters removed particles in the 0.1 micron to 3.0 micron range.
Typical operating conditions for a standard down-flow commercial gravel-bed filter are flow rates of 100-180 cfm/sq. ft. of cross section and a dirt holding capacity of 0.04-0.08 lb./sq. ft. of cross section. Most of the pressure drop is due to passage across the thin layer of filtered solids in the upper bed of the filter. Because the gravel is coarse (1/8-1/4 inch), dust smaller than 1 micron is not removed adequately.
The disadvantages of commercial upgraded media filters have suggested that an up-flow and/or down-flow filter with flow from coarse to fine media is a far more practical filtering device, but such filters present a number of problems. The greater efficiency of such filters, especially of an up-flow filter, is attributed to the classification of particles that occurs in the washing cycle of the filter: for example, in an up-flow filter the largest particles of sand settle to the bottom of the filter and the smallest at the top. Then, as the gas to be filtered is blown into the sand from the bottom, the coarser impurities are trapped in the coarser layer of sand and the finer impurities pass into finer layers of sand before being trapped.
Typical operating conditions for a graded coarse-to-fine sand-bed dust collector are flow rates of 12 to 100 ACFM/sq. ft. cross section and dirt holding capacities of up to 0.06 lb./sq. ft. cross section. The filterable solids are distributed throughout the sand and the pressure drop is likewise distributed across the entire body of sand. Thus, if the sand can be held in place, the flow rates can be much higher than in a down-flow filter for the same pressure drop, and finer media can be used so that finer dust particles can be removed.
Removal of the particles or droplets is improved if laminer or viscous flow is maintained in the filtered media. If the filter-media grains move during the filtering stage, the gas flow becomes less laminer or less viscous, and dust particles are released. High flow rates during the dust removal stage tend to lift and move the filter media grains. A number of attempts in the prior art have faced the problem of holding the sand in place during high filtering rates in an up-flow liquid filter. U.S. Pat. No. 620,621 to Veazie teaches containing the sand layer between an upper and lower foraminous diaphragm, with the upper diaphragm being connected to a device for applying pressure to sand layers to hold the sand particles close together for efficient filtering. U.S. Pat. No. 3,278,031 to Rosaen teaches a piston arrangement for compressing the filtering medium under a perforated plate, and the pressure of the piston is released to allow medium to expand for washing. U.S. Pat. No. 2,723,761 to Van Der Made et. al. teaches an up-flow filter that routes some of the liquid to the top of the filter to supply compacting pressure to the bed of filtering media. U.S. Pat. No. 3,202,286 to Smit teaches the use of an open grate across the top of the filtering media to hold the media in place through natural bridging between elements of the grate.
A problem found in graded-media up-flow or down-flow filters is that the filter rate is limited by the need to hold the top layers of the filtering media in place. The foraminous devices used on the upper layer of the media in the Veazie, Rosaen, and Smit patents will yield some of the filter material through the perforations of the hold-down device if the flow rate is high enough. Alternatively, if the hold-down device is pressurized to oppose higher filter rates, as in the Van derMade et. al. patent, there is danger that the pressurized fluid will channel to the filtrate outlet, and all filtering action will immediately cease. If a mechanical hold-down device is created with small enough perforations to physically retain the filter media against high pressure without depending upon bridging of the media particles, the hold-down device itself may become the finest layer of filtering media and be subject to rapid clogging, or in an upflow filter the upper layer of filter media may be required to be coarse enough that extremely thorough filter action cannot be achieved.
A further problem exists in cleaning a graded up-wash filter. It is desirable to loosen the beds of filter media and suspend the particles so that they can move freely, thereby releasing dirt trapped in the interstices. To accomplish this task in apparatus such as that taught in the Smit patent, it is necessary to use wash flow rates greater than filter flow rates, often requiring a separate wash pump or blower for the task. Furthermore, in all prior up-flow up-wash graded filters, the wash rate is limited according to the rate that will lift the finest particles of the filter media completely out of the filter housing. During an up-wash process, the finest media particles form a visible plane at the top of the wash flow, and the adjustment of flow rates between a rate that will retain the particles and a rate that will wash the particles away must be carefully controlled.
Ideally, the filter media should be cheap and readily available, with sand being a preferred material. In the up-wash process, it is known that the media will be self-sorting into layers graded according to the size of the particles when all particles are of similar density, the larger particles settling at the bottom of the filter and the smaller particles at the top. If in cleaning the filter the finest particles are to be retained and not blown out the top of the filter with the dirt, then there is an additional limitation that the largest particles must be of sufficiently small size that they can be lifted and suspended in the wash gas while the wash flow rate is maintained below the rate that will remove the finest particles from the top of the filter. Ordinarily, the settling rates of various sized particles of similar density are such that maximum size ratio between the largest and smallest media particles is approximately 3:1, which is not a very great range.
The present invention seeks to solve these and other problems of the prior art, as will be disclosed below.